Differential Amplifier Analysis/Design

Thread Starter

Elevon

Joined Jun 7, 2020
14
Is it possible to analyse a differential amplifier circuit by first splitting the power and ground rails that connect the transistors together and then analysing each side seperately? What would be the simplest approach to designing a differential amplifier?
 

LvW

Joined Jun 13, 2013
1,056
Is it possible to analyse a differential amplifier circuit by first splitting the power and ground rails that connect the transistors together and then analysing each side seperately? What would be the simplest approach to designing a differential amplifier?
Designing or analyzing?
In any case, power and ground connections must not be removed or "splitted".
To me, the most simple techique is to treat the diff. amplifier as a common-collecor/common-base two-stage amplifier.
 

MrAl

Joined Jun 17, 2014
7,669
Is it possible to analyse a differential amplifier circuit by first splitting the power and ground rails that connect the transistors together and then analysing each side seperately? What would be the simplest approach to designing a differential amplifier?
I have to agree with Lvw in that you can not split the two transistor sections and do them one at a time.
This is because the whole point is that the differential action occurs because of the interconnectivity of the two transistors in that they must share the ground currents from the two emitters so that one transistor can "steal" current from the other transistor. As one transistor turns on harder the other turns off a little more. You could never see that kind of interaction if you split the two up into two separate circuits.
Good question though.

There are several ways to go about analyzing this kind of circuit.
 

Papabravo

Joined Feb 24, 2006
13,939
I have to agree with Lvw in that you can not split the two transistor sections and do them one at a time.
This is because the whole point is that the differential action occurs because of the interconnectivity of the two transistors in that they must share the ground currents from the two emitters so that one transistor can "steal" current from the other transistor. As one transistor turns on harder the other turns off a little more. You could never see that kind of interaction if you split the two up into two separate circuits.
Good question though.

There are several ways to go about analyzing this kind of circuit.
I have at least one textbook example where analyzing the sections separately is done. There is a trick to it however.
 

LvW

Joined Jun 13, 2013
1,056
I have at least one textbook example where analyzing the sections separately is done. There is a trick to it however.
Perhaps I have misunderstood the phrase "splitting the power and ground rails"...?
Of course, we can split the whole circuit into two separate parts (but of course, each part with its own power and ground connection).
In principle, thats what I have proposed while saying "treat it as a two stage amplifier consisting of a common-collector stage with a load (input resistance of a common-base stage)."
Do you mean such a textbook example?
 

Thread Starter

Elevon

Joined Jun 7, 2020
14
Designing or analyzing?
In any case, power and ground connections must not be removed or "splitted".
To me, the most simple techique is to treat the diff. amplifier as a common-collecor/common-base two-stage amplifier.
Thanks for the analysis suggestion - this will prove very useful to me in future!
 

Thread Starter

Elevon

Joined Jun 7, 2020
14
I have to agree with Lvw in that you can not split the two transistor sections and do them one at a time.
This is because the whole point is that the differential action occurs because of the interconnectivity of the two transistors in that they must share the ground currents from the two emitters so that one transistor can "steal" current from the other transistor. As one transistor turns on harder the other turns off a little more. You could never see that kind of interaction if you split the two up into two separate circuits.
Good question though.

There are several ways to go about analyzing this kind of circuit.
Thank you for the response, this cleared up a misconception which was puzzling me for a while about the transistor interactions so I'm very grateful!
 

MrAl

Joined Jun 17, 2014
7,669
I have at least one textbook example where analyzing the sections separately is done. There is a trick to it however.
Hi,

Oh ok sounds interesting, and well if you care to show a little of it or just give a brief summary of what is required to it that way.
 

Papabravo

Joined Feb 24, 2006
13,939
Sure. If the emitter resistor is Re, then redraw it as two resistors of 2*Re in parallel. Now analyze half the differential amplifier with an emitter resistor of 2*Re
 

LvW

Joined Jun 13, 2013
1,056
Sure. If the emitter resistor is Re, then redraw it as two resistors of 2*Re in parallel. Now analyze half the differential amplifier with an emitter resistor of 2*Re
....without the loading effect (1/gm caused by the other transistor?)
 
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